通常情况下线程等待一个由中断服务发出异步信号的信号量。

Usually a thread waits on a semaphore that is signaled asynchronously by an interrupt handler.

youdao

注意，条件函数并不是异步信号安全的，因此不能在信号处理函数中调用。

Note that condition functions are not asynchronously signal-safe and should not be called from a signal handler.

youdao

信号就是软件中断，可以向正在执行的程序(进程)发送有关异步事件发生的信息。

Signals are software interrupts that send information about an occurrence of an asynchronous event to an executing program, or process.

youdao